1. Field of the Invention
The invention relates to a spot welding gun for the resistance welding of workpieces, including a base body and a bracket on which electrode holders carrying electrodes are arranged, wherein at least one electrode holder is fastened to an actuating means via which the electrode holder fastened thereto is displaceable together with one of said electrodes in the longitudinal direction to a further one of said electrodes, wherein a winding device including a strip is each provided for the protection of the electrodes, which strip is arranged to be displaceable relative to the electrode between the contact surface of the electrode and the workpiece, and wherein the winding device associated with the displaceable electrode is connected with the displaceable electrode so as to follow a longitudinal movement of said electrode.
The described spot welding gun is, in particular, suited to robotic applications in the resistance welding of workpieces particularly of the sheet-metal type.
At present, sheet metals or other workpieces made of aluminum alloys and magnesium as well as zinc-coated or coated steel sheets—partially in high-strength quality—are used to an increasing extent, for instance, in the car body making industry. Apart from advantages such as weight saving and good corrosion resistance, these materials or their coatings give rise to problems during joining by resistance spot welding. Above all, the tool life quality of the employed spot welding electrodes, which is strongly reduced as compared to bright steel sheet, has adverse effects. The high wear of the electrodes entails high costs due to the frequent reworking of the electrode contact surfaces and the frequent electrode exchanges resulting therefrom as well as a reduced welding quality, particularly prior to such electrode reworking or prior to an electrode exchange.
2. The Prior Art
Various spot welding guns and, in particular, so-called “C guns”, in which the components of the spot welding gun have C-shaped configurations, are known from the prior art, yet not in combination with a circulating strip that protects the electrode from soiling.
From DE 102 21 905 A1, a welding gun including a gun compensation and a method for compensating the welding gun are known. Pneumatic or hydraulic drives, servo drive systems including gears or even controllable electromagnets are used for welding gun compensation.
The disadvantage of that spot welding gun and method resides in that the electrode, as already described in the introductory part, during spot welding is in direct contact with the workpieces or sheet metals to be welded and, hence, subjected to high wear. Consequently, frequent reworking or electrode replacement is required, for which reason frequent production stops of the spot welding guns have to be taken into account.
In order to protect an electrode from being soiled or coated by the materials to be welded, it is known to insert a metal foil in strip-form between the electrode and the workpiece. In order to ensure safe strip feeding, it is necessary to prevent the strip from adhering to the contact surface of the electrode.
From DE 197 54 546 C1, a spot welding tool including a winding device for winding on and off a foil strip transversely contacting the electrode tip of the spot welding electrode is known. The spot welding tool is equipped with an annular wind-off coil for winding off the foil strip and an annular wind-up coil for winding up the foil strip, and comprises an annular driving means for the cyclical rotation of the wind-up coil. The wind-off coil, the wind-up coil and the driving means are adjacently arranged coaxially with one another around the electrode shaft or electrode holder of the spot welding electrode. A first setting roller is arranged obliquely to the electrode shaft and at a radial distance from the wind-off coil to deflect the foil strip from the wind-off coil towards the electrode tip and place it in a transverse position. Furthermore, a second setting roller is arranged obliquely to the electrode shaft and at a radial distance from the wind-up coil to deflect the foil strip from the electrode tip towards the wind-up coil and place it in a transverse position, wherein the two setting rollers are arranged in an opposed relationship laterally of the electrode shaft.
A similar configuration is known from JP 05 192 774 A, in which the wind-up and wind-off coils are likewise arranged on the electrode shaft or electrode holder, respectively.
The previously mentioned configurations according to DE 197 54 546 C1 and JP 05 192 774 A involve the disadvantage of the accessibility of the spot welding tool being strongly limited because of the arrangement of the wind-up and wind-off coils in the region of the electrode shaft or electrode holder, thus calling for a substantial enlargement of the spot welding gun in the end region and, in particular, in the welding zone. Hence, a substantially larger design of a thus configured C gun would have to be provided, which would entail a considerable increase in weight and, as a result, require the use of substantially larger-dimensioned robots. As a result, only easily accessible workpieces can be spot-welded by a spot welding tool of this type, in particular when using a C gun.
Applications of strips for the protection of electrodes in spot welding processes are further known from JP 10 029 071 A, JP 08 118 037 A, JP 04 322 886 A or JP 05 192 774 A. In those spot welding tools, the strip, which is supplied and carried off via a winding device, is positioned above the electrode, likewise as a protection against a contact of the electrode with the workpiece or structural component, so that the strip will come to lie at the workpiece or structural component during a spot welding process. The wind-up and wind-off coils in that case are arranged and fixed independently of the spot welding tool.
This involves the drawback of requiring plenty of space so as to render direct applications feasible with flat workpieces only. The use of a C gun in the automated field, particularly with robots, is not possible.
GB 571 401 A relates to an electrode assembly for a spot welding gun using a thin protection strip of copper for the protection of the electrode, which is displaceably arranged between the contact surface of the electrode and the workpiece. The winding device together with the electrode protection strip is arranged on the movable electrode holder so as to participate in the longitudinal movement of the electrode. The construction according to GB 571 401 A involves the drawback that the strip guidance laterally of the electrode occupies a large space such that the application of the welding apparatus is feasible with flat workpieces only. The accessibility is largely affected by the electrode protection strip. Moreover, the components used to guide the strip have relatively complex structures.
JP 55 141 387 A discloses a spot welding gun in which a winding device for an electrode protection strip is likewise arranged on the electrode holder. In that construction too, the strip guidance requires a relatively large space, which has adverse effects on the accessibility. The use of the spot welding gun for welding workpieces having complex geometries is, therefore, not possible.